This invention relates to an electrolytic machining system and an electrolytic method for forming recesses, bores, patterns etc. on surfaces of workpieces of sintered tungsten carbide alloys.
In general, an electrolytic machining apparatus is comprised of a machining electrode disposed opposite to a surface portion of a workpiece to be machined, and an electrolyte which circulates as a machining liquid through a small gap formed between the machining electrode and the workpiece. Means is provided for electrolyzing the electrolyte with the workpiece, as an anode electrode and the machining electrode, as an electrolyzing electrode, in order to sink or erode a predetermined portion of the workpiece's surface into a desired recess, bore, pattern or the like.
The erosion of the workpiece during this machining operation appears to result from the anodic dissolution which occurs during the electrolysis of the electrolyte. Therefore, it has been common practice to provide, as a machining source of electric power, a source of electric power derived from the rectification of commercial alternating current power, and to connect the positive terminal of the machining source to the workpiece while connecting its negative terminal to the machining electrode, thereby electrolyzing the electrolyte with power from direct current.
Also, the erosion of the workpiece is restricted to the surface portion to be machined and more particularly to that portion of the workpiece's surface which is opposite to the machining electrode which is separated from the workpiece by a very small gap, and the configuration in which the workpiece is eroded is determined by the configuration of the machining electrode. For these reasons, the machining electrode has been used ordinarily which has a configuration substantially complementary to that of the recess, bore or pattern etc. to be formed on the workpiece.
Further, as will be readily understood, the type of electrolyte used depends upon the particular material of the workpiece. The criteria for the selection of the electrolyte involves its cheapness and easy availability as well as its harmlessness for an operator and so on. Accordingly, an aqueous solution of sodium chloride has been commonly used.
With recent development of the electrolytic machining technique as above outlined, its application has been extended to the field of machining sintered tungsten carbide alloy materials, which are increasingly utilized as tool means for working various machines and devices. In this way the characteristic features of this technique may be efficiently put to practical use, namely, materials which are too hard to be machined by the ordinary machining technique can be machined easily, by an electrolytic method provided that they are electrically conductive; complicated configuration such as recesses, bores, patterns and the like can be easily formed; and workpieces can be machined at high speeds and without cracks as compared with electrospark machining techniques used for the same purposes.
It is well known that sintered tungsten carbide alloy materials are sintered materials of tungsten carbide, WC, with cobalt and/or the like as a binder or such materials having added thereto titanium carbide TiC, and/or tantalum carbide TaC. However, it has been found that the practice of the conventional electrolytic machining processes using a source of direct current, having its positive terminal connected to a workpiece and its negative terminal connected to a machining electrode and using an aqueous solution of sodium chloride as an electrolyte, so that the electrolyte is electrolyzed with power from direct current hardly leads to erosion of a tungsten carbide alloy workpiece, for reasons which will be subsequently explained.
Under these circumstances, i.e., using as an electrolyte an aqueous solution of sodium chloride and direct current, the respective metals contained in a workpiece of tungsten carbide alloy, which is now an anode electrode are oxidized as follows: EQU WC .fwdarw. WO.sub.3 EQU tiC .fwdarw. TiO.sub.2 EQU taC .fwdarw. Ta.sub.2 O.sub.5 EQU co .fwdarw. CoCl.sub.2
Among the products formed by these chemical reactions cobalt chloride CoCl.sub.2 is subsequently reacted with hydroxyl group in the electrolyte solution to form cobalt hydroxide Co(OH).sub.2 which, in turn, is dispersed into the solution. On the other hand, the remaining products, tungsten oxide WO.sub.3, titanium oxide TiO.sub.2 and tantalum oxide Ta.sub.2 O.sub.5 are not subject to such reactions and remain, adhering to the surface of the workpiece. This appears to result in the stoppage of electrolytic machining of the sintered tungsten carbide alloys.
In order to cause the electrolytic machining of sintered tungsten carbide alloy materials to proceed, the oxides of tungsten, titanium and/or tantalum must be removed from the surface of the workpiece, preferably by dissolution into the electrolyte solution. In this connection, however, it is to be noted that the electrolyte solution should be selected on the bases that it is harmless for an operator, suitable for effecting efficient machining and that it is not deteriorated during long service.
Also it is to be noted that the machining electrode used should not be consumable due to its erosion in operation. Any wear of the machining electrode in operation leads to a serious disadvantage, namely, that those portions of a workpiece's surface other than the predetermined surface portion may be eroded to form a recess, bore, pattern or the like having an undesired configuration, resulting in a decrease of the degree of machining accuracy. Further it will be readily understood that a machining source of electric power and other elements should not cause any wear of the machining electrode in operation.